Method of making a shell transformer



Dec. 6, 1966 H. T. JONES 3,289,280

METHOD OF MAKING A SHELL TRANSFORMER Original Filed Feb. 27, 1961 2Sheets-Sheet 1 [WW/776W 3'3 1 32 AM! Ida/2653 Ag f H. T. JONES METHOD OFMAKING A SHELL TRANSFORMER Original Filed Feb. 27, 1961 Dec. 6, 1966 2Sheets-Sheet 2 United States Patent 3,289,280 METHOD OF MAKING A SHELLTRANSFORMER Howard T. Jones, Hendersonville, N.C., assignor to GeneralElectric Company, a corporation of New York Application Jan. 25', 1965,Ser. No. 434,164, which is a division of application Ser. No. 91,730,Feb. 27, 1961. Divided and this application Nov. 19, 1965, Ser. No.

1 Claim. (Cl. 29-15557) This is a division of application Serial Number434,- 164, filed January 25, 1965, now Patent 3,248,781 issued May 3,1966, which in turn is a division of application Serial Number 91,730,filed February 27, 1961, now abandoned both said applications beingassigned to the same assignee as the present application.

This invention relates to reactive transformers, also known as ballasttransformers, and more particularly to a core and transformerconstruction useful for regulating the current through arc dischargedevices such as mercury vapor lamps.

The small transformer conventionally used to energize discharge lampsserves a dual function. At starting, it provides a sufficiently highvoltage to ionize or ignite the lamps; during operation, it limits thecurrent through the lamp. In order to provide the current limitingaction, such transformers are conventionally of the high leakagereactance type, that is, they are provided with magnetic shuntsproviding a low reluctance path for leakage reactance flux. The reactivetransformer construction disclosed herein, although intended primarilyfor ballasting mercury vapor discharge lamps and described principallyfrom the standpoint of such utility, are also useful in related fieldslike arc welding and voltage regulation wherein it is desired to limitthe current through a load.

The transformer portion of a regulator ballast can be made in manyconceivable ways. The basic requirements are that independent primaryand secondary electrical circuits be mutually linked by a magneticcircuit and that another magnetic circuit be provided to link either theprimary or the secondary circuit without necessarily linking the other.In addition, the secondary electrical circuit should be linked by asaturable magnetic circuit wherein the flux linkages are not a linearfunction of the primary voltage.

In the past, reactive transformers have generally been provided with amagnetic core, formed of a stack of relatively thin flat laminations ofmagnetic material. The use of grain-oriented magnetic material ortransformer iron permits greater flux density with less core loss andlower exciting current. However when grain-oriented sheet magneticmaterial is used for transformer core lamination, although the flux canbe parallel to the grain orientation in the principal direction of thecore, it is transverse to the orientation in the transversely arrangedportion of the core. Reactive transformers have also been made usingwound cores of grain-oriented magnetic material. The problem then arisesof making the maximum effective utilization of core winding techniquesto reduce the total amount of magnetic material required whilemaintaining superior electrical characteristics, and of providing inmost effective manner the shunts and other features necessary to achievethe desired electrical characteristics.

The object of the invention is to provide a reactive transformerconstruction and a method of making same wherein the most effectiveutilization is made of core and coils to reduce the total amount ofmaterial and labor or operations required while achieving superiorelectrical characteristics.

3,289,280 Patented Dec. 6, 1966 In a particular transformer constructionand winding technique according to the present invention, thetransformer core is formed by first winding a long strip of magneticsheet material flatwise layer upon layer to form a laminated ring. Thering is then shaped by flattening to an elongated straight-sided loop,that is a loop forming an elongated generally rectangular window. Thecore is then annealed for stress relief and grain growth and may at thispoint be impregnated with a suitable resin or binder whereby it willhenceforth maintain its shape. The core is then cut transversely bysawing or otherwise into substantially equal parts, each part having anelongated U-shaped. Each part, which will henceforth be referred to as awound U-core, is now used as the principal portion of the core of areactive transformer, so

, that from the original wound core, two separate transformers willresult.

To complete the reactive transformer, the U-core is provided with asaturable area of reduced cross section by sawing a slot partiallythrough the core normal to the edge of the lamination, preferably in thebight or transverse portion of the U. Wound primary and secondaryelectrical windings are then assembled inductively on the U-core in sucha way that the secondary windings are at the closed end of the core loopwhere the restriction occurs. An end member or yoke is placed across thelegs of the U to provide a closed magnetic circuit around a centralwindow in such way that the butt joints occur next to the primarywinding and are in the primary portion of the magnetic circuit. Reactiveshunts are then introduced between the primary and secondary portions ofthe core loop by placing a stack of laminations on one or both sides ofthe core loop spanning the central opening so that the laminations ofthe reactive shunts are edge to edge with those of the U-core- Toprevent magnetic saturation of adjacent laminations, layers of paper orother composition may be inserted be tween the magnetic shunts and theU-core. The yoke and the reactive shunts may be fastened in place by asuitable strap.

A reactive transformer formed in the foregoing manner makes veryefficient use of magnetic material since all stock is initially in theform of strip, no irregular punchings productive of waste are used, andwinding and cutting operations are reduced to a minimum. The uniqueplacement of the component parts wherein the air gap inevitablyintroduced by the yoke is located next to the primary windings andwherein the restriction is located next to the secondary windings,results in superior electrical characteristics under the limitationsunavoidably introduced by the winding technique. The invention thusprovides a comparatively inexpensive reactive transformer with nosacrifice in performance. In fact, the wound U-core reactivetransformers in accordance with the invention achieve a higherefficiency and show lower losses than conventional transformers using astack of punched laminations.

For further features and advantages and for a better understanding ofthe invention, attention is now directed to the following description ofpreferred embodiments and to the accompanying drawings. The features ofthe invention believed to be novel will be more particularly pointed outin the appended claim.

In the drawings:

FIGS. 1a and 1b illustrate, to a smaller scale than the other figures ofthe drawings, the core of a transformer made according to the inventionat two different stages.

FIG. 2 is an isometric view of a loosely shunted transformer constructedin accordance with the invention.

FIG. 3 is a plan view, with the electrical windings sectioned, of thetransformer illustrated in FIG. 2.

FIG. 4 is an equivalent circuit for a regulator ballast for arcdischarge lamps illustrating the electrical characteristics of areactive transformer according to the invention.

FIG. is a plan view similar to FIG. 3 but with the core structurepartially cut away and illustrating a variant of the present invention.

Referring to the drawings and more particularly to FIG. 1a, the corestructure of a reactive transformer according to the invention is madeby winding a long strip of grain-oriented magnetic sheet materialflatwise layer upon layer until a sufiicient cross-sectional area isbuilt up and a laminated ring 1 is formed as shown. The laminated ringis then reformed, for instance by pressing in a suitable clamping jig orvise, to an elongated loop 2 having parallel sides defining arectangular window 3 as illustrated in FIG. lb. Alternatively, the stripof magnetic sheet material may be wound directly on a suitable mandrelto a rectangular loop form. The laminations are next annealed by heattreatment to achieve stress relief and grain growth, the stress reliefpermitting the shape to be maintained; additionally, to preventsubsequent separation of the laminations, the loop may be impregnated atleast in part with a suitable resin or binder which is allowed to set orpolymerize. The rectangular loop is then cut along a single plane 4, 4transverse to the long sides, into two equal U-shaped portions 2a, 2b.The loop may be severed by any suitable means such as a metal saw but,in order to prevent short circuits between laminations, it is preferredto use a milling cutter or a thin abrasive wheel.

In accordance with the invention, where a core type reactive transformeris to be made, one of the U-shaped parts 201, or 2b illustrated in FIG.lb is used as the main portion of the core. A transformer so constructedis illustrated in FIGS. 2 and 3. Before assembling the windings on theU-core, a restriction in the cross-sectional area of the core is createdin order to provide a saturable area in that portion of the core loopwhich is to carry the secondary winding. This is most conveniently donein the bight or transverse portion of the U by cutting a narrow slot 5across one edge of the lamination to a depth of approximately /3 thewidth of the lamination. Next pre-wound and preformed electricalwindings are slipped into place over the legs of the U-core. A pair ofsecondary windings 6, 7 is slipped on'first so as to be at the closedend of the U-core next to the slot 5. Then the primary windings 8, 9 arepositioned near the open end of the U-core legs. The windings arepreferably insulated from the core by a layer of insulating materialsuch as the paper insulators 10 shown.

The main magnetic circuit linking the primary and secondary winding iscompleted by means of a yoke 11 which is butted up against the ends ofthe legs of the U-core so as to bridge the gap. The laminations of theyoke are arranged as illustrated to meet edge to edge with those of theU-core but with the plane of the laminations in the yoke transverse tothat of the laminations in the U-core. An insulating layer 12a, suitablya thin sheet of paper, or a resin coating, or the oxide on thelaminations is interposed between the core and yoke at the butt joints12 to prevent a continuous electrical circuit around the core. Anadvantageous feature in accordance with the invention resides in thatthe butt joints 12 formed by juxtaposing yoke 11 to the ends of theU-core 2a occur in that part of the main magnetic circuit bearing theprimary windings. Butt joints are unavoidably present in the wound coreconstruction adopted and, by placing them in the primary portion of themain magnetic circuit, the losses which they introduce are kept to aminimum. The effective air gap at the butt joints affects the primarymagnetizing reactance and the gap may be adjusted, for instance byinserting additional paper separators, to control the input powerfactor. The laminations of the yoke are held together by means of frameplate 13 and the yoke is fastened to the U-core 2a by means of a steelstrap 14 whose ends are clamped together in conventional fashion by acrimping band 15.

In order to provide a magnetic circuit linking either the primary or thesecondary windings without necessarily linking them both together, amagnetic shunt is added to the basic core loop between primary andsecondary windings. As illustrated, the reactive shunt consists of apair of stacks of laminations 16, 17 which are spaced from thelaminations of the core by thin insulating spacers 13, 19. Thelaminations of the reactive shunt are placed against the side of themain magnetic loop in edge to edge relationship with the laminations ofthe main loop. The reactive shunts span the central opening through thecore, thus providing a leakage path for the primary and secondaryportions of the core loop. The entire reactive transformer assembly isordinarily mounted in a suitable container or can and impregnated with apotting compound; prior to such potting, the shunts may be held in placeon the core loop by tape (not shown) wound for example about the centeror ends of the stacks of laminations 16, 17. By placing the reactiveshunts consisting of stacks of laminations beside the wound core loopinstead of in the same plane, and by orienting its laminations edge toedge with those of the main core loop, an effective low loss shunt isobtained. The losses with this arrangement are substantially lower thanthey would be if the shunt were placed in the plane of the main coreloop which would entail that most of the shunted flux would necessarilyhave to pass through several adjacent layers of the laminations in themain core loop in a direction normal to the laminations.

A reactive transformer serving as a regulator ballast for a gaseousdischarge device such as a mercury vapor lamp or a fluorescent lamp, isintended to maintain a nearly constant current into the lamp load withvariations of supply potential. When the lamp is operated about at itsrated power, variations in lamp current are reflected in the energyoutput of the lamp even though the potential across it remains nearlyconstant. The regulator ballast performs its function by supplying asubstantially constant voltage to the lamp and by limiting the amount ofcurrent which the lamp is permitted to draw. The present reactivetransformer is intended to provide a non-linear transformer couplingbetween a source of power and a load circuit including a dischargedevice such as an arc lamp, and a series capacitor. The seriescapacitor, in addition to its current limiting function, provides thenecessary leading current through the non-linear transformer coupling sothat a constant output voltage may be obtained. The factors involved maybe explained by reference to the schematic diagram of FIG. 4 showing theequivalent circuit of a reactive transformer according to the inventionconnected as a regulator ballast for an arc lamp 20. The elementsenclosed within the dashline rectangle represent the effective impedanceelements resulting from the transformer construction which has beendescribed. These elements include an ideal transformer 21 having a turnsratio of preselected value. Reactance 22 represents the primarymagnetizing reactance referred to the secondary circuit. Reactances 23and 24 represent, respectively, the secondary magnetizing reactance andthe total leakage reactance. The output circuit is completed by theconnection of a shunt capacitor 25 across the output leads and a seriescapacitor 26 in series with the load consisting of arc lamp 20.

With the transformer construction which has been described, it ispossible to vary each of the effective reactances to obtain optimumcharacteristics for a given installation. The reactance shunts 16, 17placed between the primary and secondary core provide the effectiveleakage reactance 24 which must be proportioned to the series capacitorreactance 26. The value of this leakage reactance may be adjustedwithout altering the other effective reactances, by varying the crosssectional area of the laminations in the reactive shunts and the widthof the air gap between the edges of the reactive shunts and the edges ofthe core loop. The effective secondary magnetizing-reactance 23 providesa non-linear saturating effect that affects both the regulation of loadcurrent with variations in input potential and the crest factor of theload current. This reactance is controlled principally by the corerestriction placed on the secondary side of the main core loop. Theshape of the restriction, whether hole or slot, and also its dimensionscontrol the actual characteristics of the secondary magnetizingreactance. Normal variations in the primary supply voltage affect onlythe degree of saturation in the secondary magnetic circuit so that thesecondary magnetizing reactance remains substantially constant and thusthe voltage output of the ballast likewise remains nearly constant.

By placing the butt joints 12 in the primary portion of the core loop,they alfect only the primary magnetizing reactance 22, and the effectivegap may be adjusted to control the input power factor. If the buttjoints were located in the secondary portion of the core loop, theywould affect also the secondary magnetizing reactance, thereby adverselyaffecting regulation and increasing transformer losses. Shunt capacitor25, which may also be termed a peaking capacitor, is used to increasethe peak open circuit voltage above the turns ratio voltage and shouldpreferably be of a value to resonate with the leakage reactance 24 atslightly above the third harmonic frequency of the supply voltage.Although shown as connected across the entire output of the ballasttransformer, the peaking capacitor may also be connected across only apart of the secondary turns to reduce to some extent the open circuitvoltage supplied by the ballast.

A variant of the invention is shown in FIG. 5 illustrating a shell typereactive transformer. In this construction, a pair of U-cores 2a, 2b arepositioned side by side and the common or center leg of the resultingshell-type core is constituted by two juxtaposed legs, one from eachU-core. The single secondary winding 31 is slipped over the center legso as to be located next the closed or inner end of the U-cores. Thecore restriction, in this instance is provided by holes 32, 33 drilledthrough the transverse portions or bights of the U-cores, being therebylocated in the secondary portion of the main core loop. A single primarywinding 34 is slipped over the center leg so as to be located next tothe butt joints 35 Where the open ends of the U-cores are bridged by ayoke 36 consisting of a stack of laminations. The plane of thelaminations in the yoke 36 is transverse to that of the laminations inthe U-cores, in similar fashion to the embodiment of FIGS. 2 and 3, andthe laminations are likewise arranged for edge to edge contact. Thereactive shunt is provided by two stacks of laminations, only one ofwhich, 37, is shown in the figure, spanning the windows of both coreloops and located between the primary and secondary windings. Thereactive shunts are arranged in similar fashion to those in FIGS. 2 and3, being placed edge to edge on the side of the main core loop.

The shell type reactive transformer construction of FIG. 5 makes use oftwo core loops but only one set of primary and secondary windings.Evidently, for a given transformer rating, the cross sectional areas ofthe two U-cores which make up the complete core will be equal to halfthat of the core of the transformer in FIGS. 2 and 3. Thus in thisconstruction, both parts 2a, 2b of an elongated rectangular core loop asillustrated in FIG. 1b are used in a single reactive transformer buteach core part is of half the cross sectional area. The shell type coreconstruction offers most of the features of the transformer designillustrated in FIGS. 2 and 3 along with the added advantage of animproved outward heat transfer from the core because of a greaterexposed core area.

Although certain preferred embodiments have been illustrated anddescribed herein, it is to be understood that these are intended asexemplary and not limitative of the invention. Modifications inconstruction and method will readily occur within the scope of thepresent teachings, to competent designers. The appended claim isintended to cover any such modifications falling within the true spiritand scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

The method of making a shell-type reactive transformer comprisingwinding a band of magnetic material fiatwise layer upon layer to form anelongated straightsided core, cutting said core in two on a plane midwaythrough the elongated sides to form a pair of substantially equal sizedU-shaped core portions; arranging the two U-shaped core portions side byside to form a double U with a central core leg and opposite side legshaving their end edges all substantially in the same plane; placing onthe central core leg a secondary winding adjacent the closed end of thecore and a primary winding adjacent the open end so that the centralcore leg projects beyond said windings at the open end, closing the openends by providing a yoke bridging the end edges of all said core legs toprovide closed magnetic loops around a pair of windows, said yoke beingformed of a flat stack of laminations placed with their edges abuttingthe end edges of all said core legs and with their planes at rightangles thereto, whereby the junctures of the laminations of the corelegs with those of the yoke form butt joints next to said primarywinding, and placing on said double U-shaped core a reactive shuntformed of at least one flat stack of laminations of magnetic materialagainst the side of the core so as to span said windows, the laminationsof the double U-shaped core and of said reactive shunt being placed inedge-to-edge relationship and with their planes at right angles to eachother.

References Cited by the Examiner UNITED STATES PATENTS 1,992,822 2/ 1935Granfield 336213 2,324,634 7/1943 McCready 336 3,128,443 4/1964 Hermanet al 336160 FOREIGN PATENTS 1,198,453 12/1959 France.

JOHN F. CAMPBELL, Primary Examiner.

R. W. CHURCH, Assistant Examiner.

